The use of polyalpha-olefins and copolymers thereof to reduce the effect of friction (“drag”) experienced by a liquid hydrocarbon flowing through a hydrocarbon transportation pipeline is well-known in the art. Reduction of the drag decreases the amount of energy needed to accomplish such flow, and therefore also decreases the costs associated with pumping. These materials, often called drag reducing agents (DRAs), can take various forms, including certain polymers in oil soluble suspensions, emulsions, pellets, gels, microfine powders and particulate slurries. However, particulate slurries that comprise ground polymers are often the least expensive form. The ultimate goal is a DRA that rapidly dissolves in the flowing hydrocarbon and that has a polymer content sufficient to ensure that the desired level of drag reduction is achieved.
The polymers that are most commonly used in preparing DRAs are polyalpha-olefins of carbon chain lengths ranging from 2 to about 40. Typically these polymers are prepared using Ziegler-Natta catalysts and frequently also co-catalysts such as alkyl aluminum compounds. These polymerization reactions tend to be very efficient, producing relatively high yield when carried out in bulk. However, they also tend to be highly exothermic. The exotherm itself creates problems which reduce the usefulness of the product if the exotherm is not effectively alleviated. These problems include, but are not necessarily limited to, a substantial reduction in the polymer molecular weight. Such molecular weight loss can result from even relatively minor deviations from a preselected optimal temperature, and can substantially reduce the efficacy of the polymer in a drag reducing agent formulation.
Those skilled in the art have attempted to reduce or otherwise control this exotherm in order to improve the quality of the polymers being produced. Some attempts to accomplish this have included, for example, carrying out the polymerization reaction in specially designed reaction bottles, wherein reactor layers ostensibly provide a level of protection from oxygen and water as potential sources of polymerization catalyst contamination.
Another method of addressing the exotherm problem for DRA polymers has been to use a screw conveyor as the reactor. This method is, however, ill-suited to producing larger quantities of polymer per batch, since in such cases a large screw conveyor must be used to achieve a commercially acceptable polymerization time. Unfortunately, the larger the conveyor, the less effective is the heat transfer, and the less effective the heat transfer, the poorer the temperature control, and hence, the quality of the final DRA polymer.
Thus, it would be desirable if a method for producing drag reducing agent polymer could be developed that preferably enables relatively tight control and/or overall reduction of the exotherm resulting from polymerization, such that high quality DRA polymers, as well as other polymers which would otherwise encounter similar undesired exotherm problems, can be produced in bulk.